The present invention relates to a liquid crystal display device using liquid crystal materials of chiral smectic C phase or sub-phase such as a ferroelectric liquid crystal or anti-ferroelectric liquid crystal thereof.
In the trend of higher definition and larger size of display screen of a liquid crystal display device, there is a strong demand for improvement of response speed and viewing angle of liquid crystal. One of the methods of meeting the demand is to use liquid crystal materials of chiral smectic C phase or its sub-phase.
As representative examples, ferroelectric liquid crystal or anti-ferroelectric liquid crystal are applied in the active matrix type liquid crystal display devices.
This kind of liquid crystal material is, however, known to induce spontaneous polarization, and to lower the holding voltage due to depolarization field when the response time of ferroelectric liquid crystal is longer than the writing time as a result of shortening of the writing time along with the trend of higher definition (see Hartmann: J. Appl. Phys. 66, 1132 (1989)). This lowering of holding voltage means so-called writing shortage, which leads to lowering of effective applied voltage and decline of contrast ratio, and large practical problems are effected.
Incidentally, in the case of alternating-current drive for driving in positive and negative symmetric modes by inverting polarity of the applied voltage in every frame, after a certain frame, when the absolute value of the signal voltage is changed largely, the holding voltage cannot follow up the signal voltage accurately. Hence, in the several subsequent frames, the brightness of the image corresponding to the signal voltage cannot be obtained, and while repeating lightness and darkness departing from the original brightness of the image, it finally settles at the stationary quantity of transmission light, which is known as step response (see Verhulst et al.: IDRC '94 digest, 377 (1994)). When such step response occurs, it is recognized as an after-image like a ghost with a tail, which is a practical problem.
On the other hand, in the case of an asymmetric mode, that is, direct-current drive, step response does not occur, and the contrast ratio is enhanced (see Tanaka et al.: SID '94 digest, 430 (1994)). However, as compared with the alternating-current drive, it is known to be lower in the response speed. The cause of decline of response speed is that writing is insufficient by one writing only, that is, that the holding voltage is lowered, and the response speed is lower as the writing time is shorter. In direct-current drive, the contrast ratio and response speed of image are in trade-off relation, and an optimum design is necessary to obtain sufficient values in both, but the available margin is narrow. Besides, problems of image sticking due to impurities or after-image due to residual hysteresis can be hardly solved by improving the driving method.
Thus, in either symmetric mode (alternating-current drive) or asymmetric mode (direct-current drive), writing shortage due to lowering of holding rate induces serious problems practically.
As measures against lowering of holding rate on the characteristic aspect of the liquid crystal material, increase of response speed and decrease of spontaneous polarization can be considered. The problems can be solved by using a liquid crystal material sufficiently high in speed and shorter in response time than in writing time in low voltage drive or in a temperature range slightly lower than the ordinary temperature, but the liquid crystal material satisfying these conditions is not known at the present. It is also regarded less likely, even in future, to realize high response speed in low temperature region, in particular.
The liquid crystal display device is further demanded to be larger in display size and higher in definition, and it is consequently required to shorten the writing time per line. Therefore, owing to the limit in enhancement of speed in liquid crystal material, it is hard to solve the above problems.
Still more, decrease of spontaneous polarization leads, in principle, to decline of response speed, and hence the problems are not solved. Thus, improvement of characteristics by liquid crystal material is insufficient as the measure for solving the problem of decline of holding voltage.
Alternatively, it may be considered to improve by the driving method or circuit structure. First, a method of increasing the storage capacitance may be considered. The storage capacitance value of active matrix type liquid crystal display element using an ordinary TN liquid crystal is nearly same as the capacity value between the pixel electrode and counter electrode filled with liquid crystal, but by increasing it 10 times or more, the problem of decline of holding voltage can be solved. However, as far as the response speed of the liquid crystal is as low as the present level, the problem of step response is not solved. Besides, along with increase of the storage capacitance, the current value also increases accordingly, and the power consumption is increased, and the load of the drive circuit is larger. It is hence not suited to practical use, and applications are limited.
As other means of solving, a method of writing a voltage near 0 V just before writing, and resetting by erasing or canceling the previously held electric charge is known. An active matrix driving method using TFT (thin film transistor) or TFD (thin film diode) is proposed in Jpn. Pat. Applin. KOKAI publication No. 7-64056, and in these methods, a part of writing time is assigned for reset action. Accordingly, the problem of step response is solved, but the practical writing time is shorter unless the number of lines is decreased, and sufficient improvement of contrast is not expected. Or, if the writing time is shortened due to heightened definition, the writing time becomes much shorter due to reset action, and writing shortage becomes a serious problem. In these methods, a sufficient reset time is not provided, and resetting is only imperfect, and hence it is impossible to eliminate step response completely. In particular, when changed from dark state to bright state, the luminance of the first frame is too high.
In order to obtain a sufficient reset time and reset completely, a circuit structure having two pieces each of TFD and signal line per pixel is reported (see Verhulst et al.: IDRC '94 digest, 377 (1994)). In this reported example, it is possible to reset while writing in other line. However, the number of elements and number of wires per pixel are too many and the driving waveform is complicated, and there are problems in the manufacturing yield and cost. Besides, with the TFD, it is hard to suppress fluctuations of the element characteristics of the entire liquid crystal display device, and it is not suited to practical use.
Moreover, Jpn. Pat. Applin. KOKAI publication No. 8-15671 discloses a liquid crystal display device in which reset lines are provided parallel to signal lines, and pixel electrodes and reset lines are connected with TFT. In this example, however, excessive reset wires are impeding enhancement of definition, and the aperture ratio is lowered and hence the luminance is lowered.
As described above, in the conventional active matrix type liquid crystal display device using ferroelectric liquid crystal or anti-ferroelectric liquid crystal, so far, no constitution is known to satisfy all of contrast ratio, image response speed, aperture ratio, and luminance at high levels.
It is hence an object of the present invention to provide a liquid crystal display device having high contrast ratio and high luminance, without causing after-image or uneven display, by using ferroelectric liquid crystal or anti-ferroelectric liquid crystal as liquid crystal material.